Digital imaging systems typically include one or more lenses and a digital image sensor. The digital image sensor captures light from an object or scene being imaged through the lenses and converts the light into electronic signals. The electronic signals are digitized and stored as digital image data in a semiconductor memory. Such digital imaging systems are used in a variety of consumer, industrial and scientific applications to produce still images and/or video, including mobile telephones, digital still image and video cameras, webcams, and other devices.
Most modern image sensors are complementary metal oxide semiconductor (CMOS) image sensors or charge-coupled device (CCD) image sensors consisting of a two-dimensional pixel array. A modern digital image sensor can include millions of pixels to provide high-resolution images.
The quality of digital images, including still and video images generated by a digital imaging system, can depend upon a variety of factors. In digital imaging systems with a wide-angle imaging lens, such as a fisheye lens, lens distortion can significantly affect the quality of the digital images. Lens distortion causes straight lines in a scene or object being imaged to appear curved in the image. The most common forms of distortion are radially symmetric, arising from the symmetry of the lens. Radial distortion can be classified as one of two main types: barrel distortion and pincushion distortion. Barrel distortion is common in images captured by wide-angle lenses, while pincushion distortion is often present in images captured by zoom or telephoto lenses.
In barrel distortion, image magnification decreases with distance from the optical axis. The effect is an image which appears to have been mapped around a sphere, or barrel. In pincushion distortion, image magnification increases with the distance from the optical axis. The visible effect is that lines that do not go through the center of the image are bowed inwards, toward the center of the image, like a pincushion. Complex distortion, which is a combination of barrel distortion and pincushion distortion, starts out as barrel distortion close to the image center and gradually turns into pincushion distortion toward the image periphery.
In conventional digital imaging systems, the error generated by lens distortion can be corrected by signal processing carried out in an image signal processor (ISP) formed on a semiconductor chip die. The ISP receives the digital image data from the digital image sensor, which is also typically formed on a semiconductor chip die. To carry out the correction of the distorted digital image data, a large buffer memory is used to store several hundred lines of the distorted image data. The required memory is sufficiently large that it cannot be included as part of the ISP chip. As a result, an additional memory chip, e.g., a dynamic random access memory (DRAM) device, is required. The additional device impacts the size and cost of the digital image system. Also, the additional device introduces delays in accessing the memory and complicates memory access bandwidth issues.